Headlights shut off randomly

My friends 87 pickup started having a weird headlight problem. When he has his headlights on they will somtimes shut off for about 2-3 sec then turn back on. All the other lights stay on, just the headlights go out. Ive gone around and checked connections and grounds and it still does it. Any help would be appreciated.

Easy test is next time they go out, pull back on the combo switch and if the lights come on (flash mode) then the factory relay, fuses and wiring are fine. Flash mode uses separate contacts in the combo switch, so that is the only change between normal and flash mode.